(1.) Field of the Invention
This invention relates to a light-radiant heating furnace, and more specifically to a light-radiant heating furnace equipped with an air-cooling system for its lamps so as to avoid overheating of the lamps.
(2.) Description of the Prior Art
Among a variety of apparatus adapted to carry out heat treatments therein, light-radiant heating furnaces in which light radiated from an incandescent lamp or lamps is irradiated onto objects or materials to be treated (hereinafter referred to merely as "objects") for their heat treatment have the following merits:
(1) Owing to an extremely small heat capacity of an incandescent lamp per se, it is possible to raise or lower the heating temperature promptly;
(2) The heating temperature can be readily controlled by controlling the electric power to be fed to the incandescent lamp;
(3) Since they feature indirect heating by virtue of light radiated from their incandescent lamps which are not brought in contact with the objects, there is little danger of contaminating objects under heat treatment;
(4) They enjoy less energy consumption because full-radiation-state operation of the lamps is feasible a short time after turning the lamps on and the energy efficiencies of the lamps are high; and
(5) They are relatively small in size and inexpensive compared with conventional resistive furnaces and high-frequency heating furnaces.
Such light-radiant heating furnaces have been used for the heat treatment and drying of steel materials and the like and the molding of plastics as well as in thermal characteristics testing apparatus and the like. Use of light-radiant heating furnaces have, particularly recently, been contemplated to replace the conventionally-employed resistive furnaces and high-frequency heating furnaces for carrying out certain semiconductor fabrication processes which require heating, for example, diffusion processes of dopant atoms, chemical vapor deposition processes, annealing processes for healing crystal defects in ion-implanted layers, thermal treatment processes for activation, and thermal processes for nitrifying or oxidizing the surfaces of silicon wafers. As reasons for the above move, may be mentioned the incapability of conventional heating furnaces for use of heating larger-sized objects uniformly, thereby failing to meet the recent trend toward larger semiconductor wafer size in addition to such advantages of light-radiant heating furnaces that objects under heat treatment are free from contamination, their electric properties are not deleteriously affected and the light-radiant heating furnaces require less power consumption.
Light-radiant heating furnaces have various merits as mentioned above and have found wide-spread commercial utility in the industry. However, conventional light-radiant heating furnaces are accompanied by such shortcomings that they are unable to heat objects of large sizes uniformly to high temperatures at high heating speeds. Namely, each lamp is equipped with a sealed envelope made of silica glass or the like and forms a point or line light source. It thus cannot make up by itself a plane light source which extends two-dimensionally. Therefore, it may be able to heat a very small area to high temperatures but is unable to heat a large area to high temperatures. For the reasons mentioned above, a plurality of lamps are arranged in a conventional light-radiant heating furnace. However, it is necessary, from the practical viewpoint, to avoid any highly-concentrated arrangement of a number of high power lamps since, when lamps are disposed close to one another, their envelopes become hotter and their service lives become extremely short as their outputs increase. As a result, the object is heated unevenly due to non-uniform irradiation intensity and the object may develop a certain deformation, when the object has a large size. Furthermore, such a conventional light-radiant heating furnace cannot increase the intensity of irradiating energy in its irradiation space. Thus, the possible upper limit of the heating temperature is as low as about 1200.degree. C., leading to such drawbacks that it is unable to effect an intended heat treatment to any satisfactory extent or otherwise it requires a longer treatment time period to achieve a necessary heat treatment.
The above-mentioned drawbacks will become serious problems where precisely-controlled heating is required, particularly, in the heating step of semiconductor fabrication for instance.